It is conventional in the nuclear reactor art, and in particular in high temperature gas cooled reactors, to provide a reactor core made up of a number of core blocks or elements which are stacked in columns. The core elements may include fuel elements and control rod elements. An example of such a fuel block or element is disclosed in U.S. Pat. No. 4,060,450 which illustrates and describes a fuel element or block comprised substantially of graphite and having generally cylindrical passages or channels therethrough for receiving fuel rods or rod segments and facilitating passage of coolant through the fuel element. Other blocks or elements are for accommodating control rods and are similar in exterior shape but generally include channels for control rods, reserve shutdown pellets or power rods which include a neutron absorbing material, such as boron carbide, and function to control operation of the reactor core.
In high temperature gas cooled reactors employing prismatic fuel and control elements having generally planar end surfaces, gaps may form at the interface between stacked elements allowing coolant leakage flow into and out of the element coolant channels. Should a gap be created between adjacent fuel elements, the coolant leakage into and out of the nearest coolant channels can result in undesirable maldistribution of coolant within the fuel elements.
In order to prevent or at least substantially reduce such coolant leakage, it has been proposed to provide a dependent peripheral flange on one end surface of the element and a mating recess on the other to align the coolant passages of the stacked elements and form a peripheral seal. For further information regarding the structure and operation of such fuel elements, reference may be made to U.S. Pat. No. 3,413,196, particularly to FIG. 5. The effectiveness of the seal is, in part, a function of the thickness of the flange. However, as flange thickness increases, the relative core power density decreases because a continuation of the normal fuel hole and coolant passage pattern is not possible in the region underlying the flange without degrading the operation of the seal.